docs: def released(self, unit, lock, timestamp):
          '''Called on the leader when it has released a lock.
  
          By default, does nothing but log messages. Override if you
          need to perform additional housekeeping when a lock is released,
          for example recording timestamps.
          '''
          interval = _utcnow() - timestamp
          self.msg('Leader released {} from {}, held {}'.format(lock, unit,
                                                                interval))